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Date: Wed, 26 Sep 2012 12:55:41 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: Konstantin Belousov <kostikbel@gmail.com>
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Cc: freebsd-hackers <freebsd-hackers@FreeBSD.org>
Subject: Re: monitor+mwait and volatile-ish
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on 26/09/2012 12:10 Konstantin Belousov said the following:
> On Wed, Sep 26, 2012 at 11:14:41AM +0300, Andriy Gapon wrote:
[snip]
>> So what's my point. - using volatile variable with cpu_monitor requires
>> DEVOLATILE to silence compiler warning about discarding volatile; this is
>> unnecessary code bloat - adding volatile cast in the checks is easy to
>> forget and adds code bloat
>> 
>> Possible improvements: - make the argument of cpu_monitor be 'const
>> volatile void *', the most permissive type; this would also be a hint
>> that variable should be volatile - add some magic dust to cpu_monitor
>> that would tell compiler to not cache the variable; right now I can only
>> think of the "memory" constraint, but it seems to be too big of a hummer
>> 
>> What do you think about this?
> 
> You might claim that the asm writes to *addr by specifying it in the output
> constraint. This should fool the compiler into reload *addr after the
> monitor execution.
> 

You mean something like:
static __inline void
cpu_monitor(const void *addr, u_long extensions, u_int hints)
{

	__asm __volatile("monitor"
	    :  "=m" (*(char *)addr)
	    : "a" (addr), "c" (extensions), "d" (hints));
}

This seems to do the job with base gcc, 4.6, 4.7 and clang.
Thank you!
-- 
Andriy Gapon